一、重發訊息有兩個原因：

①接收方未收到，此時重發是應該的。②接收方收到訊息但是發回的應答包丟失了，此時重發訊息則重複了，所以在接收方還得新增一個機制：若收到的訊息與以前發過的訊息重複，則再次傳送應答包。在這些機制下，能夠初步的保證UDP傳輸的完整性。為了UDP包不在IP層上被分片，所以設計UDP包每個不得大於乙太網的MTU.採用相互握手重發

二、直接看程式碼

public void run() {

try{

String local_path = Environment.getExternalStorageDirectory().getPath() +"/gateway_update_file/"+ file_name;

File local_file =new File(local_path);

int size =new Long(local_file.length()).intValue();//檔案總的大小

System.out.println("檔案總的大小"+" = "+ size);

if(size ==0) {

//當前檔案為0，重新獲取檔案

//........

return;

}

//SEND_SIZE此變數為每包傳送的大小（跟接收返回的傳送包大小）

PACKETS= (size /SEND_SIZE) +1;//跟接收方定義的接收總的包數（即傳送多少包）

InetAddress inetAddress = InetAddress.getByName(GW_IP_ADDRESS);

if(socket==null) {

socket=newDatagramSocket(null);

socket.setReuseAddress(true);

socket.bind(newInetSocketAddress(Constants.UDP_PORT));

socket.setSoTimeout(3000);

}

inti;

int count =1;

FileInputStream fin =new FileInputStream(local_file);

byte[] bytes =new byte[SEND_SIZE];

while((i = fin.read(bytes,0,SEND_SIZE)) != -1) {//迴圈讀取檔案

byte[] send_byte = GatewayCmdData.FileDataToGatewayCmd(PACKETS, count, bytes);//此byte陣列為與接收方定義的資料結構體

DatagramPacket data =newDatagramPacket(send_byte, send_byte.length, inetAddress, Constants.UDP_PORT);

socket.send(data);

Log.i(TAG+"傳送的byte = ", Arrays.toString(bytes));

while(true) {

byte[] rec_byte =new byte[128];

DatagramPacket getack =newDatagramPacket(rec_byte, rec_byte.length);

socket.receive(getack);

if((int) MessageType.A.SEND_UPDATE_FILE_TO_GATEWAY.value() == rec_byte[11]) {過濾掉無用資料

ParseGatewayData.ParseUpdateCountData startUpdateData = new ParseGatewayData.ParseUpdateCountData();

startUpdateData.parseBytes(rec_byte);解析接收到的資料包含傳送的資料包次數（即第幾包）

if(count == startUpdateData.packet_count) {//如果返回的資料包次數與傳送包次數一致，則傳送下一包

if(startUpdateData.packet_count==PACKETS) {//最後確認包（即已經完成相關操作）

String crc32 = GatewayInfo.getInstance().getGatewayUpdateCRC32(context);

byte[] bt = GatewayCmdData.FinallyUpdateCmd(crc32);

DatagramPacket packet =newDatagramPacket(bt, bt.length, inetAddress, Constants.UDP_PORT);

socket.send(packet);

Log.i(TAG+" = ", TransformUtils.bytesToHexString(bt));

System.out.println("傳送最後一包= "+ TransformUtils.bytesToHexString(bt));

booleanisRun =true;

while(isRun){

socket.receive(getack);

if(rec_byte[32] ==0){

System.out.println("更新成功");

fin.close();

if(socket!=null){

socket.close();

最後關閉socket、檔案流

}

isRun =false;

DataSources.getInstance().GatewayUpdateResult(0);

}else{

System.out.println("更新失敗");

DataSources.getInstance().GatewayUpdateResult(1);

}

Log.i(TAG,"最後一包返回值= "+ Arrays.toString(rec_byte));

}

}

System.out.println("傳送下一包= "+ count);

count++;

break;

}

}else{

Thread.sleep(1000);

System.out.println("未接收到返回重新發送= "+ count);

socket.send(data);

}

}

}

}catch(Exception e) {

e.printStackTrace();

}finally{

System.out.println("finally = "+"UpdateHelper");

}

}